bgp: T3460: add verify() steps for local-as override
bgp: T3460: add verify() steps for local-as override

It is only possible to set one local-as override per BGP neighbor/peer-group.
In addition to this, the override AS number is not allowed to be the same as
the one from the global BGP process.

If this would still be the case frr-reload would error out:
 > frr-reload output: 184 % Cannot have local-as same as BGP AS number

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x